The present invention relates to high frequency, high voltage power supplies for gaseous luminous tube lighting of the type commonly found in commercial and decorative home and commercial applications. Such lighting may be of either the neon or mercury type, or both, depending on the colors desired. More specifically, the present invention relates to an improved dimmer apparatus for controlling the intensity of such luminous tubes.
Most conventional high frequency neon power supplies operate at a fixed current output determined by power supply design and the length of the neon tube or tubes connected thereto. Such supplies are, in short, operated at a single output level corresponding to full or maximum light intensity.
While fixed full-output neon supplies are satisfactory for most applications--usually for outdoor or window advertisement applications--there is growing demand for lower or variable intensity neon signage principally for indoor applications where normal high intensity illumination does not comport with the subdued and darkened atmosphere associated with many food and beverage establishments--common users of neon signage. The present invention, therefore, pertains to a dimmer arrangement for high frequency neon power supplies that permits the continuous adjustment of light output from full intensity down to a low light output level of, for example, about 5-10% thereof.
In certain instances a conventional SCR or triac-type `conduction angle` or pulse width modulation (PWM) lamp dimmer may be employed to vary the light intensity, particularly where the neon sign is powered from a standard 60 Hz power transformer supply. And it might reasonably be assumed that the PWM dimming scheme could be extrapolated to high frequency neon power supplies as this is the principle upon which many high frequency switching power supplies operate.
Several problems, however, have been encountered when applying PWM dimmer technology to high frequency neon power supplies. These include the non-uniform illumination of the neon tube and the lowering of the output voltage below that necessary to assure neon gas excitation--both phenomena occurring at lower illumination intensities.
As presently understood, the reason for the first of these limitations is related to the distributed tube capacitance which may be as high as 50 picofarads or more. This capacitance progressively shunts tube current to ground along the length of the tube, that is, as viewed by moving from the respective tube ends toward the center. As the voltage across the tube is substantially independent of tube current (actually, the negative resistance characteristic of the neon tube results in a slightly increasing tube voltage with lowering tube currents), this capacitive leakage current is also substantially independent of tube illumination or dimmer setting. For a 20 KHz neon supply and typical neon tube, this current is approximately 12 milliamperes.
By comparison, a neon tube current of about 25 milliamperes is typical for normal (full) neon tube illumination. As these two current components (i.e. tube leakage and tube illumination currents) are in quadrature, a total supply current of under 28 ma results. Thus, it will be appreciated that the leakage current causes only a negligible reduction in neon tube current for normal tube illumination intensities and consequently this gradual current reduction along and toward tube center produces a correspondingly trivial reduction in light intensity.
This is not the case for lower tube illumination intensities, however. Take, for example, a tube dimming of 80%, that is, where the desired tube current is 20% of full tube intensity current or 5 ma. For this configuration (i.e. quadrature leakage and illumination currents of 12 and 5 ma, respectively) the total supply current is computed to be 13 ma. It should be observed, however, that the full 13 ma supply current enters the neon tube ends as all of the capacitive leakage and tube neon currents flow through these points. Thus, the tube ends are illuminated not by a mere 5, but a 13, milliampere current.
The current through the center section of the tube (which is at "ground" potential by reason of the balanced nature of the supply output), however, is the previously specified 5 ma--the 12 ma quadrature leakage current having been fully shunted to ground. The tube is therefore illuminated to a 5 ma intensity in the center, but gradually increases to 13 ma at the ends. This differential produces a clearly visible and objectionable illumination non-uniformity that only gets worse as greater dimming levels are selected.
The second limitation of PWM neon supplies relates to the intrinsic low pass filter characteristic of the power supply and neon load. This filter characteristic--which has a cut-off frequency generally of twice the supply operating frequency--is created by the series equivalent inductance of the high voltage transformer working against the secondary capacitance and the previously mentioned tube leakage capacitance.
The oscillator output waveform, for ordinary `full output` operation, is of generally symmetrical form having substantial energy at the fundamental or operating frequency. Thus, the above-mentioned low pass characteristic is of minimal consequence for ordinary operation. However, as the pulse widths are narrowed by the PWM circuitry (as occurs upon dimming with this conventional approach), the relative fundamental energy content of the resulting output waveform drops dramatically. And by reason of the above-discussed low pass filter characteristic, the remaining high frequency harmonic energy is not coupled to the neon tube and therefore does not significantly contribute to the available excitation voltage thereof. As dimming is increased (i.e. as the pulse widths narrow) the neon tube excitation voltage may drop below the requisite ionization potential thereby resulting in erratic and unreliable tube operation, specifically, the failure of the tube to illuminate or an oscillatory-type flickering or blinking thereof.
It must be emphasized at this juncture that the above-described low pass characteristic, while fatal to PWM dimming, is central to the present invention. An important distinction is that in the PWM dimmer the narrowed pulses are utilized in an attempt to achieve illumination (albeit, at a reduced intensity) while in the present invention the narrowed pulses contribute no illumination, but are used solely to maintain residual ionization. The illumination intensity of the present dimmer is determined by the `duty cycle` or `on` time of full output, normal frequency and width pulses. This latter `full output` dimming technique being a form of Pulse Group Modulation (PGM).
Applicant previously developed a luminous tube dimmer employing the principle of Pulse Group Modulation ("PGM") in which full amplitude high frequency pulse groups were generated at relatively low frequency repetition rate. The intensity, or dimming, was controlled by adjusting the number of high frequency cycles comprising each pulse group. This approach was described in U.S. application Ser. No. 980,539 filed on Nov. 13, 1992, now U.S. Pat. No. 5,349,273. As noted in that application, certain anomalies associated with the transient turn-on phase of each pulse group required special treatment in order to obtain satisfactory ground fault interruption ("GFI ") and over-voltage protection ("OVP") capability. Specifically, GFI operation had to be `blanked` or inhibited during this transient phase of each pulse group in order to preclude false GFI sensing. While this approach has proved satisfactory, it nonetheless represents a compromise in GFI operation.
A second problem encountered with PGM (in particular with the sharp rise-time of each pulse group) relates to acoustic noise or `clicking`. As currently understood, these clicks are caused by slight mechanical movements of the transformer core or windings and result in an annoying buzz at the low frequency PGM repetition rate.
The present invention, by contrast, employs a combination of shifting the `energy` of the high frequency oscillator (upwardly) and generating a `soft` transition between the normal and shifted oscillator modes to provide for dimming without the above-noted problems. More specifically, the present high frequency oscillator is never turned-off, rather, its energy is shifted upwardly in frequency an amount sufficient to take advantage of the inherent multi-pole low-pass characteristic defined by the intrinsic (and unavoidable) load and supply reactances.
Thus, the oscillator--although superficially operating normally--nevertheless provides a substantially reduced excitation to the load during such `shifted` intervals whereby only a minimal amount of tube illumination occurs. Yet, the high frequency oscillator is still operational and generating sufficient excitation to preionize the luminous tube load thereby greatly diminishing transient over-voltage and GFI problems at the commencement of each non-shifted `on` cycle. To further minimize generation of false GFI and over voltage signals, oscillator shifting is slowed, that is, gradually moved between its two frequency extremes over, for example, a 400-800 .mu.Sec period. It will be appreciated that the degree of dimming may therefore be set by correspondingly adjusting the duty cycle of the respective `normal` and `shifted` energy modes of the high frequency oscillator. Typical `normal` and `shifted` frequencies of operation are about 20 KHz and 40-50 KHz, respectively.
To further take advantage of the above-described low pass filter effect, the duty cycle of the high frequency oscillator is altered, simultaneously with the upward shift in frequency, to a less symmetric `square wave` (i.e. one having successive `half-cycles` of progressively disproportionate duration). This latter effect causes an increase in the harmonic content of the oscillator output (comparative to the fundamental component) which, in turn, results in even less energy being passed to the luminous tube during these `shifted` periods. As a consequence, a typical luminous tube current of 30 milliamperes drops to about 5 milliamperes under the above-described frequency/waveform shifts.
A further feature of the present invention is the employment of integration on the high frequency oscillator frequency control input whereby the frequency of this oscillator transitions smoothly between its normal and shifted modes (and visa versa). By reason of the above-noted intrinsic `low pass` contour, these smoothed frequency transitions result in correspondingly smoothed power supply output amplitude changes which, in turn, eliminating the sudden electronic impulses believed responsible for the objectionable clicking and buzzing noises.
It is therefore an object of the present invention to provide an improved neon luminous tube dimmer that does not exhibit the annoying clicking and buzzing noises found in connection with certain pulse group modulation dimming arrangements. It is an object of the present dimmer to employ periodic upward shifting of the oscillator frequency/energy--working into the intrinsic low pass response associated with the high frequency transformer and luminous tube load--to effect a substantial reduction in luminous tube current and reduction in light output and to selectively adjust the percentage of time that the oscillator is `shifted` to thereby correspondingly adjust the degree of dimming. It is a further object to enhance the dimming function by altering the high frequency oscillator waveform to thereby augment the upward shifting of the oscillator output by reason of increasing the percentage harmonic content thereof. It is yet another object to control and slow the rate of transition between the oscillator `shifted` and `un-shifted` modes to thereby minimize the generation of annoying acoustic clicking and buzzing.